I only have 13 bps and I plan on starting to breed rats soon. The cost of F/T rats isnt too bad as long as you have the available funds for it. The cost of breeding rats is cheaper from what I understand, however from what I've heard, the time and hassle of actually doing is what deters most from doing it. Also though, once you've got a good enough breeding colony going you can always euthanize and freeze your extras(for later use or sale) or sell them as pets for a little extra cash.
Hmm i may actually give it a try soon. Nothing to big. Obviously not enough to feed my collection but maybe starting up a small group to see how much time/effort a small group takes to breed.